The present invention relates to a method and apparatus for power amplification and a system incorporating the same. The invention is particularly, but not exclusively, intended for use in a wireless communications base station.
Known wireless communications systems require provision of a high power signal for transmission in order to ensure reception over a suitably wide area. This is achieved by amplifying the data signal to be transmitted, typically at a wireless base station itself, and feeding the resulting amplified signal to the antenna array for wireless transmission. The high amplification levels required for such applications can lead to non-linear distortion of the data signal, which is undesirable. It is therefore highly desirable to use high-performance, linear power amplifier architectures in such applications.
A problem with known linear Power Amplifier (PA) technology is that a static back-off, proportional to the signal statistics (Complementary Cumulative Distribution Function (CCDF)), is required to obtain linearity in the system. This back-off decreases the maximum obtainable efficiency of the system and drives mechanical, thermal, and overhead powering costs for wireless transmit architectures. Known techniques yield PA efficiencies in the order of 10-12% for high power designs for signals with high peak-to-mean ratios such as Code Division Multiple Access (CDMA) and Wideband CDMA (W-CDMA). It is therefore desirable to improve the efficiency of power amplifiers so as to reduce operating costs.
Known solutions include Envelope Elimination and Recovery (EER) but such solutions have limited performance due to cross modulation induced by varying the DC voltage applied to the device output. EER cannot be easily incorporated in a wideband architecture since the cross modulation ultimately limits the system linearity and hence limits the ability to meet current standards requirements. Significant memory components are added to the signal envelope at wide bandwidths thus creating an inherently non-linear system.
Power amplifiers today account for a high proportion of wireless Basestation Transmitter System (BTS) costs. In addition, infrastructure support for the power amplifiers (in terms of mechanical, cooling, and DC powering), combined with the basic power amplifiers costs, total a majority of BTS costs. It is therefore desirable to identify means whereby to reduce installation and/or operating costs of such systems.
According to one broad aspect, the invention provides a power amplifier arrangement comprising: a power amplifier for receiving an input signal and amplifying said input signal to form an output signal, the power amplifier having an output capacitance which varies; a power supply for supplying a power supply voltage to the power amplifier which is modulated proportionally with respect to changes in the input signal; a variable capacitor having a variable capacitance which in combination with the output capacitance results in an adjusted output capacitance, wherein the variable capacitor is controlled so that the adjusted output capacitance is substantially constant.
Some embodiments further comprise a signal processing unit for receiving said input signal and for providing a first control signal to the power supply, said signal processing unit forming said first control signal responsive to said input signal; wherein the power supply modulates the power supply voltage in response to the first control signal so as to vary said power supply voltage proportionally with respect to changes in the input signal.
In some embodiments, a signal processing unit is adapted to generate a second control signal to the variable capacitor which controls the variable capacitance of the variable capacitor for the purpose of maintaining the adjusted output capacitance to be substantially constant.
In some embodiments, an input signal to the power amplifier is time delayed to allow the power supply voltage to be modulated in time with amplification of the input signal by the power amplifier; and the second control signal is time delayed, synchronous with the input signal to the power amplifier, to allow the substantially constant adjusted output capacitance to be provided even while the output capacitance of the power amplifier is changing due to a change in operating power supplied to the power amplifier caused by modulation of the power amplifier power supply by the input signal.
In some embodiments, the variable capacitor is located at an output stage of the power amplifier.
In some embodiments, an impedance matching network is connected at the output of the power amplifier to provide impedance matching to an output impedance of the power amplifier.
In some embodiments, the adjusted output capacitance is a function of the output capacitance, as well as a function of a capacitance of the variable capacitor, such that the adjusted output capacitance=f(output capacitance, capacitance of the variable capacitor), and wherein the capacitance of the variable capacitor is controlled such that the adjusted output capacitance is substantially constant.
Some embodiments further comprise a predistortion module for providing a predistorted version of the input signal to the power amplifier, said predistortion module receiving from the signal processing unit predistortion coefficients for distorting said input signal, said predistortion coefficients being formed by said signal processing unit responsive to said input signal.
In some embodiments, the signal processing unit receives a feedback signal from at least one of the power amplifier and the power supply and wherein said first control signal formed by the signal processing unit is formed using at least one of the input signal, a power supply feedback signal and a power amplifier feedback signal.
In some embodiments, the variable capacitor is a high power, high Q-factor device.
In some embodiments, the variable capacitor is a voltage controlled device.
In some embodiments, the voltage controlled device is a micro-electromechanical system (MEMS) device.
In some embodiments, the variable capacitor is a current controlled device.
In some embodiments, the current controlled device is a micro-electromechanical system (MEMS) device.
Some embodiments further comprise a digital-to-analog converter, a low pass filter and an operational amplifier, wherein the signal processing unit is adapted to generate the second control signal based on a lookup table means and supply the second control signal to the variable capacitor, via serial succession of the digital-to-analog converter, the low pass filter and the operational amplifier placed between the signal processing unit and the variable capacitor, which controls the variable capacitance of the variable capacitor so that the adjusted output capacitance is substantially constant.
In some embodiments the variable capacitor is a micro-electromechanical system (MEMS) device comprised of a flexible top plate, a fixed bottom plate, and at least one support post to support the flexible top plate.
According to another broad aspect, the invention provides a method of operating a power amplifier arrangement, comprising the steps of: providing an input signal to a power amplifier, the power amplifier amplifying the input signal to form an output signal, the power amplifier having an output capacitance which varies; providing a power supply voltage to the amplifier to enable the power amplifier to amplify the input signal; modulating the power supply voltage to the power amplifier in response to a first control signal received by the power supply from a signal processing unit, said signal processing unit forming said first control signal responsive to the input signal, wherein the power supply modulates the power supply voltage so as to vary said power supply voltage proportionally with respect to changes in the input signal; providing a second control signal from the signal processing unit to a variable capacitor having a variable capacitance which in combination with the output capacitance results in an adjusted output capacitance, said signal processing unit adapted to generate the second control signal which controls the variable capacitance of the variable capacitor so that the adjusted output capacitance is substantially constant; and time delaying inputting of the input signal to the power amplifier in order to allow the power supply voltage to be modulated in time with amplification of the input signal by the power amplifier.
In some embodiments, the method further comprises the step of time delaying inputting the second control signal, synchronous with the input signal to the power amplifier, to the variable capacitor so that the variable capacitance of the variable capacitor is controlled for the purpose of maintaining the adjusted output capacitance to be substantially constant.
In some embodiments, the method further comprises the step of predistorting the input signal so as to provide the power amplifier with a predistorted version of the input signal for amplification.
In some embodiments, the input signal is predistorted by a predistortion module which receives predistortion coefficients generated by the signal processing unit, said predistortion coefficients being generated by the signal processing unit responsive to the input signal.
In some embodiments, the method further comprises providing to the signal processing unit a feedback signal from at least one of the power amplifier and the power supply and wherein the first control signal is formed by the signal processing unit utilizing at least one of the input signal, a power supply feedback signal and a power amplifier feedback signal.
In some embodiments, the variable capacitor is a voltage controlled device.
In some embodiments, the variable capacitor is a current controlled device.
In some embodiments, the variable capacitor is a micro-electromechanical system (MEMS) device.
According to another broad aspect, the invention provides a wireless communications base station transmitter including a power amplifier arrangement.
According to another broad aspect, the invention provides a communications network including a power amplifier arrangement.
According to another broad aspect, the invention provides a method of amplifying an input signal using a power amplifier having a power supply input, the method comprising the steps of: receiving the input signal; amplifying the input signal using the power amplifier to provide an output signal, the power amplifier having an output capacitance which varies; modulating the power supply input to the power amplifier in accordance with a first control signal responsive to said input signal, wherein the method includes the step of time delaying inputting of the input signal to the power amplifier to allow the power supply input to be modulated in time with amplification of the input signal by the power amplifier; and providing a second control signal to a variable capacitor having a variable capacitance which in combination with the output capacitance results in an adjusted output capacitance, wherein the second control signal controls the variable capacitance of the variable capacitor so that the adjusted output capacitance is substantially constant.
In some embodiments, the method further comprises the step of time delaying inputting the second control signal, synchronous with the input signal to the power amplifier, to the variable capacitor so that the variable capacitance of the variable capacitor is controlled for the purpose of maintaining the adjusted output capacitance to be substantially constant.
In some embodiments, the variable capacitor is a voltage controlled device.
In some embodiments, the variable capacitor is a current controlled device.
In some embodiments, the variable capacitor is a micro-electromechanical system (MEMS) device.
According to another broad aspect, the invention provides a program for a computer for controlling a power amplifier arrangement comprising a power amplifier, a power supply, and a signal processing unit, the program comprising code to carry out the steps of: providing an input signal to the power amplifier, the power amplifier amplifying the input signal to form an output signal, the power amplifier having an output capacitance which varies; providing a power supply voltage to the power amplifier to enable the power amplifier to amplify the input signal; modulating the power supply voltage to the power amplifier in response to a first control signal received by the power supply from a signal processing unit, said signal processing unit forming said control signal responsive to the input signal, wherein the power supply modulates the power supply voltage so as to vary said power supply voltage proportionally with respect to changes in the input signal; providing a second control signal from the signal processing unit to a variable capacitor having a variable capacitance which in combination with the output capacitance results in an adjusted output capacitance, said signal processing unit adapted to generate the second control signal which controls the variable capacitance of the variable capacitor so that the adjusted output capacitance is substantially constant; and time delaying inputting of the input signal to the power amplifier in order to allow the power supply voltage to be modulated in time with amplification of the input signal by the power amplifier.
In some embodiments, the program further comprises code to carry out the step of time delaying inputting the second control signal, synchronous with the input signal to the power amplifier, to the variable capacitor so that the variable capacitance of the variable capacitor is controlled for the purpose of maintaining the adjusted output capacitance to be substantially constant.
According to another broad aspect, the invention provides a method of providing a signal transmission service over a communications network including a power amplifier arrangement.